1. Field of the Invention
This invention relates to a data communication system that preferably uses bi-directional communication links between network nodes to send frames of data.
2. Description of the Related Art
Modern data communication networking can be traced back to at least the late 1950s and early 1960s with the introduction of the Telecommunication carrier or T-carrier system by Bell Labs for communicating digitized voice streams around the world. The personal computer revolution triggered the introduction of a variety of computer network technologies, including ARCNET, Token Ring, Ethernet, and FDDI, in the late 1970s and early 1980s, with Ethernet becoming the dominate computer networking protocol over time. As the demand for wide area data communication grew, the telecommunications industry responded in the late 1980s and early 1990s with the introduction of technologies such SONET to carry the ever growing data traffic along with the traditional voice streams.
With the growth of personal computers in the early 1980s came the need to interconnect them with peripherals such as printers and modems, which lead to the standardization of PC serial and parallel ports. As the number and complexity of PC peripherals grew, protocols such as Universal Serial Bus (USB) from Intel and Firewire (IEEE 1394) from Apple were introduced in the middle 1990s to support the new functionality, such as Audio/Video (A/V) devices, and to reduce the number of connectors needed on a PC.
The need for real-time data communication networks for controlling components in automobiles came with the introduction of fuel injection systems in the early 1980s. Many car makers and suppliers responded with proprietary protocols, such as Control Area Network (CAN) from Germany, Vehicle Area Network (VAN) from France, and J1850 from the US, with CAN becoming the dominate control protocol over time.
During the early 1990s increasing demand for information and entertainment (infotainment) in vehicles with the associated increase in bandwidth and streaming capabilities lead to the development of protocols such as Multi-Media Link (MML) from Delphi and ST Microelectronics, and Media Oriented System Transport (MOST) from Oasis Siliconsystems and then SMSC. At the same time, Texas Instruments was promoting an automotive variant of IEEE 1394; however, over time MOST became the dominant protocol in the automotive infotainment market.
Although a wide variety of networking technologies from markets such as telecommunications, computer, and consumer electronics were available at the times, the automotive industry developed and selected unique networking technologies such as CAN and MOST due to special automotive requirements, such as robustness, performance, quality, and limited transmission distance. Many of the features and capabilities of networks such as CAN and MOST incorporated ideas from other markets, but fine tuned them for automotive applications. For instance, CAN uses the same shared bus topology as the original coaxial cable based Ethernet; however, arbitration was improved to make it more deterministic.
The latest automotive networking technology to be introduced is FlexRay, which has been developed and promoted by German and US car makers targeted at next generation “X by wire” applications. These applications refer to systems such as braking and steering being controlled by digital communications instead of traditional mechanics and hydraulics. Such a system needs high throughput and low latency, and must be deterministic and fault tolerant. The FlexRay protocol achieves these requirements, but at significant cost, since redundant cabling provides the fault tolerance.
The FlexRay topology is based on a redundant active star. All nodes have 2 ports, which connect to 2 different active stars, which then connect to all other nodes providing single point fault tolerance. Each link to the active star is single twisted pair wire capable of bi-directional communication and data sent from one node is broadcast to all nodes. The active star topology with bi-directional cabling and broadcast messaging is the same as the original ARCNET. The redundant star is new with FlexRay.
In the FlexRay protocol, the highest level in the communication timing hierarchy is a communication cycle that repeats at a fixed rate, which is preferably set by one timing reference. All other nodes have free running clocks that are periodically synchronized to the timing reference, which results in some jitter on the local network clocks. The communication cycle is divided in a static segment and a dynamic segment for communicating data frames. The static segment is time division multiplexed with each timeslot dedicated to a particular node for high priority deterministic communication. The dynamic segment provides shared bandwidth for lower priority, which is allocated in an implicit token ring manner to provide deterministic behavior.
Such fixed rate communication cycles with time division multiplexing dates back to the late 1950s with the introduction of T1. The concept of dividing the cycle into static and dynamic segments was seen as early as the early 1990s with introduction of Firewire and USB and may have existed early in the telecom market. FlexRay's way of dividing between static and dynamic segments, however, is closer to MOST than Firewire or USB. Something that appears to be new with FlexRay is the implicit token ring arbitration for the dynamic bandwidth.
FlexRay was developed for X-by-wire applications since other automotive networks such as CAN and MOST did not fulfill the necessary requirements for bandwidth and deterministic behavior (in the case of CAN), and fault tolerance (in the case of MOST). The CAN protocol typically uses a twisted pair cable shared by all nodes, which can send and receive messages, but not simultaneously. Messages are sent serially on the bus starting with an ID and are sensed by all nodes. If multiple nodes start sending at the same time, the message with the dominant ID wins arbitration and finishes sending the message. Other nodes with non-dominant IDs stop transmission until the dominate message has been sent. The CAN protocol has some fault protection in that if one of the two conductors are shorted to power or ground, communication over the other conductor is still possible. CAN is not appropriate for X-by-wire applications since the data rate is too low, less than 1 Mbit/sec, and there are no dedicated timeslots for high priority communication.
A MOST network typically uses a ring topology of uni-directional point to point links. The physical layer can be optical or twisted pair wire and supports data rates from 25-150 Mbits per second. Like a FlexRay communication cycle, a MOST frame repeats at a fixed rate and is divided into time division multiplexed fields for dedicated data sources and shared fields that all nodes can arbitrate for. Unlike FlexRay and advantageous to MOST, MOST nodes are synchronized by PLLs to the timing source node, which minimizes jitter on the local network clocks. Unlike FlexRay and advantageous to FlexRay, MOST did not provide any fault tolerance at the time. Unlike FlexRay and advantageous for some types of communication but not others, the time division multiplexed field of MOST only supported raw streaming data while the TDM fields of FlexRay only transport packets addressable to one or more destinations. Additionally, nodes arbitrate in a token ring manner for bandwidth in the dynamic segment of a FlexRay communication cycle, while MOST uses a message priority based arbitration scheme somewhat similar to CAN.
MOST, which was initially introduced with an optical physical layer, traces its roots to the telecom world of T1 and SONET (Synchronous Optical NETwork), which was first proposed to the standards organization ANSI in 1985. Like SONET, MOST has a ring topology with uni-directional point to point links and a repetitive frame structure that is time division multiplexed into a number of channels. In both SONET and MOST all nodes are precisely synchronized by PLLs at the bit level timing to a single timing reference, which is unlike any of the computer or computer peripheral networks mentioned except possibly for IBM's Token Ring, which is not time division multiplexed.
For both SONET and MOST, precisely synchronized clocks at all network nodes facilitate the transport of any data streaming from an analog to digital (A/D) converter at a source to a digital to analog (D/A) converter at a destination. Additionally, TDM channels dedicated to A/Ds synchronized to either the SONET or MOST bit timing provide a raw data transport mechanism with zero overhead.
Although both SONET and MOST can allocate one or more TDM channels for data packet communication, MOST has low level mechanisms, such as arbitration, acknowledgement, error detection, and system control, that are appropriate for local area networks but not the wide area telecommunications networks that SONET services. Although implemented differently, message arbitration and acknowledgement in MOST packet communication channels can be traced back to the early computer communication protocols such as ARCNET and IBM's Token Ring. The primary innovation of the automotive MOST protocol was the specific combination of particular ideas from the telecom and computer worlds to produce a simple and cost effective networking technology focused on the needs of the target automotive market.
Although MOST has plenty of bandwidth for X-by-wire applications and has the dedicated timeslots that provide guaranteed bandwidth, FlexRay was developed primarily because MOST did not provide fault tolerance at the time. Secondly, MOST's TDM channels communicate raw streaming data like SONET instead of packets like CAN. Just like with SONET, MOST could provide single point fault tolerance with a redundant communication channel traveling in the opposite direction of the main ring, however, like FlexRay, this doubles the cost of the physical layer.